CAN we always trust our GIRL FRIEND?

Depends on who you pick for a girlfriend :) If you choose someone for their looks alone and don't look for substance in their principals and morals, you will always live in doubt. Having said that, there are girls who do have looks and good nature, but looks alone should not weigh while deciding on a girlfriend. At the end of the day, its much more important to feel proud of a dutiful, caring and genuine person than someone you can just show off and then worry about.

Sinerity is not a measurable qualilty thus it becomes very diffilut for anyone to comment on someone's integrity. We can make it up on our own with the passage of time whether the person is sincere to us or not? Trust comes later as we progress in a relation and when we find that the other person is interested in our life and she shares the same feelings then it leads to trust.
It is worth saying that Trust is like a coin you can see from one and on one side only. If you the other side then you may feel cheated or distrusted but that is not always. Thus, keep faith in yourself nobody will ever try to cheat you. Love one that will make the trust more and more stronger.